1. Area of the Art
The present invention is related to systems for detecting target ions in body fluids, and more specifically, it is related to plasticizer free ion sensors, and sensors containing ion exchangers.
2. Description of the Prior Art
Throughout this application, various references are referred to within parentheses. Disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains. Full bibliographic citation for these references may be found at the end of this application, preceding the claims.
Carrier-based ion-selective electrodes (ISEs) and optical sensors (optodes) have been used for detecting target ions in body fluids for many years. ISEs produce a measurable electrical change upon contact with a fluid sample containing target ions. Optodes, thin film ion-specific optodes or particle-based optodes, typically contain a target ionophore and an indicator ionophore. The target ionophore complexes with the target ion when present, and the indicator ionophore provides an indication of such complexing, such as by a color change.
Traditionally, poly(vinyl chloride) (PVC) has been the polymer matrix most commonly used in membrane-based ISEs and hydrophobic bulk optodes(1). This is primarily due to its high tensile strength, chemical inertness, and plasticizer compatibility(2).
There are several disadvantages, however, associated with the use of plasticized PVC in ion-selective sensors, one of which is plasticizer leaching (3). For in vivo measurements, where biocompatibility is of paramount importance, plasticizer leaching may result in a serious inflammatory response(4). Furthermore, leaching of plasticizers in biomedical devices has recently prompted the issuance of a health warning on biomaterials containing the common plasticizer bis(2-ethylhexyl) phthalate (DOP), and the commencement of an initiative to eliminate or reduce plasticizer content in such devices(5). With respect to ion determinations, exudation of plasticizer is one of the principle factors directly affecting sensor lifetime(6)(7). Moreover, a reduction in plasticizer content also results in an increased membrane resistance(6), and it may also decrease the solubility of the active sensing components within the membrane, thus causing a marked decrease in sensitivity and selectivity(8). These effects are expected to become more pronounced in miniaturized sensing platforms.
Miniaturized optodes that function in accordance with bulk extraction principles have typically been either fiber optic or particle-based. Optical fiber-based optodes are usually fabricated by immobilizing a sensing layer on the distal end of an optical fiber by a simple dip-coating procedure. Sensors of this type have been developed for several clinical analytes, including H+,(9) Cl−,(10) Na+,(11) and K+(12). Although this approach offers the advantages of reduced sample volume and high signal-to-noise ratio, it does not appear feasible for multiplexed analysis.
Particulate optodes have been produced by several different approaches, such as heterogeneous polymerization techniques(13)(14), solvent casting(15), and very recently with a high-throughput particle generator(16)(17). An obvious advantage of particulate optodes is their ability to independently interrogate a sample and produce a distinguishable analytical signal. To date, particle-based optodes have been used for very innovative applications, including flow cytometry(17) and intracellular monitoring(13)(18). The lifetime of these sensors, however, still remains a concern. In particulate probes used for intracellular measurements, lifetimes have been reported as short as 30 minutes(13), thus validating the need for methods that improve sensor lifetime.
One approach that has received a substantial amount of effort is the fabrication of plasticizer-free polymers. Several such polymers have been evaluated in ISEs or ion-selective field effect transistors (ISFETs), including polyurethanes(19), polysiloxanes(8)(10), silicone rubber(21)(22), polythiophenes(23), epoxyacrylates(24), and methacrylic(25) and methacrylic-acrylic copolymers(26)(28). Polymers synthesized via free radical initiated mechanisms, such as methacrylic-acrylic copolymers, appear quite attractive because of the numerous polymerization methods and infinite monomer combinations available to create polymers with a diverse range of physical and mechanical properties [29]. Hall and coworkers have done a substantial amount of work in this area (28 to 30, 78). Particularly, in PCT application WO 00/54039, Hall et al describe a selective polymer material with an acrylate backbone and a plurality of pendant lipophilic plasticising groups. Acrylate monomers are used to synthesize the polymers. The polymers are self-plasticising and thus are plasticizer free.
There are several disadvantages, however, to working with polymers containing acrylic monomers, including increased susceptibility to acid and base hydrolysis [30], inferior tensile strength [31], increased tacticity [30, 31], and malodorous vapor, all relative to their methacrylic counterparts. Therefore, a need still exists to develop a polymer matrix that would overcome the above-discussed disadvantages.
Carrier-based ion-selective electrodes (ISEs) and optical sensors (optodes) may also include ion-exchangers for improving their ion selectivities. Tetraphenylborate derivatives have been used as ion-exchangers in cation-selective solvent polymer membrane electrodes and bulk optodes for many years. They were initially introduced into ISE membranes to facilitate Donnan exclusion, which is the electrostatic repulsion of lipophilic anions trying to extract into the membrane(61). In addition to reducing anion interference, tetraphenylborates also decrease membrane resistance(62). It was later found that the presence of such borates in optimized concentrations improves ionophore selectivity by stabilizing the concentration of ion-ionophore complex(61). The delocalized monoanionic charge that these compounds possess, in combination with their sterically hindered molecular structure make them very weakly coordinating. This is a characteristic that leads to weak, non-specific ion pair formation and maximum ionophore-mediated selectivity of the membrane(63).
Because the unsubstituted tetraphenylborate (TPB-) is susceptible to decomposition via acid hydrolysis, oxidants, and light, the search for more chemically stable derivatives began many years ago(61)(62)(64). The most successful derivative thus far has been the highly substituted 3,5[bis-(trifluoromethyl)phenyl]borate (NaTFPB)(65). The presence of strong electron-withdrawing groups decreases the tendency for cleavage of the boron-phenyl bond, because the amount of localized charge at the ipso carbons is significantly reduced(64). Furthermore, the presence of electron-withdrawing groups suppresses the π-coordination of the phenyl groups, thus making the compound more inert, and improving its electrochemical stability by increasing the reduction potential(64). Even though halogenated derivatives, such as NaTFPB, are more lipophilic and more resistant to phenyl cleavage, acid hydrolyzed decomposition still occurs albeit at a much slower rate(63)(65). This shortcoming limits the use of tetraphenylborates in systems requiring an acidic sample pH, as in the case of heavy metals, such as Pb2+(66).
Compounds that may be suitable alternatives to tetraphenylborates are carboranes, specifically closo-dodecacarboranes. The chemical structure of the perbromo-substituted derivative appears in FIG. 8. These compounds possess many characteristics that may make them suitable ion-exchangers. Very weak ion pair formation is observed due to the lack of electron lone pairs and π-electrons, a property rarely found in anions(67). Moreover, as stated by Reed, the dodecacarborane anion, CB11H12, behaves “like a 3D analogue of benzene” because of the versatile functionalization chemistry that these compounds possess(67). The desired lipophilicity of this class of carboranes can easily be tailored both at the boron vertices(64)(69–71) and at the carbon vertex(70)(71). The most lipophilic derivatives that have been synthesized are those of the perhalogenated(68) and peralkylated dodecacarborane anion(69).
In addition to potentially unparalleled lipophilicity, the carboranes possess many other characteristics that make them suitable for electrochemical applications. For example, they are not susceptible to acid and base hydrolysis and they are relatively inert to electrochemical oxidation(˜2.0 V vs. ferrocene/ferrocenium at Pt in dichloromethane)(67). High Ih symmetry and tangentially delocalized σ-bonding make the carboranes one of the most chemically stable classes of compounds in chemistry. Furthermore, their bulky size (nearly 1 nm in diameter) and sufficient charge delocalization meet the criteria imposed for sufficient ion-exchanging. Another advantage, important for bulk optode studies, is their lack of absorption in the UV-Vis spectrum. Therefore, it is desirable to further study the carboranes for developing a more robust ion-exchanger to be used in chemical sensors.